This invention relates to output windows of vacuum electron devices.
For vacuum electron devices with circular multi-mode output waveguide designs, the output window normally consists of one or more layers of dielectric, at least one of which will be joined to the device's output waveguide in a vacuum-tight bond, usually achieved by brazing the dielectric to the metal waveguide. While a single layer output window gives excellent transmission at a sequence of defined wavelengths, to achieve broadband performance, a multi-layered window should be used.
Theoretically the bandwidth performance of a single half-wavelength-thick window design can be improved via the use of quarter wavelength transformers abutting the two faces of the window in order to match the window impedance to the free space impedance. To space the triple layers of the window takes this concept one step further, and spaced triple windows have been proposed for electron tubes. Thin ceramic layers may be spaced from the central half-wavelength-thick window to form “matching” cavities. These cavities enable the bandwidth of the window to be significantly extended beyond 20% (a range extending from 10% below the center frequency to 10% above the center frequency) with a return loss of better than −25 dB. This is true for both single and multi mode circular waveguides.
The invention is especially concerned with output windows for gyrotron-travelling wave tube electron devices, although it is also applicable to other broadband vacuum electron devices.
Referring to FIG. 1 of the accompanying drawings, which is a schematic axial cross-section of a known gyrotron-travelling wave tube (gyrotron-TWT) with a conventional broadband output window, and also to FIG. 2, which is a schematic axial cross-section of the output window on an enlarged scale (turned through 90 degrees), the gyrotron-TWT depicted in FIG. 1 consists of a waveguide 1 which is the interaction region between an electron beam from an electron gun 2 and an input rf electromagnetic wave, launched along waveguide sidearm 3, it is desired to amplify. The electron beam undergoes a helical path along the waveguide 1 under the influence of solenoid 4. The waveguide 1 is evacuated, one end being closed by a wall 5 and by another wall (not shown) behind the electron gun 2, and a flared region 6 connects the other end to an output waveguide 7, which is sealed by a conventional triple output window, indicated generally by the reference numeral 8, from which the amplified rf signal is launched.
The interior of the waveguide 1 may be provided with a helical corrugation (not shown)—“Gyro-TWT with a Helical Operating Waveguide: New Possibilities to Enhance Efficiency and Frequency Bandwidth”, Gregory G. Denisov, Vladimir L. Bratman, Alan D R Phelps and Sergei V Samsonov, IEEE Transactions on Plasma Science, Vol. 26, No. 3, June 1998. In view of the broadband nature of the output, a spaced triple layer window is used. For optimum performance the design should be symmetrical about the central window. The performance of such a design is very sensitive to dimensional variations, with spacing and ceramic thickness tolerances of tighter than ±0.05 mm necessary to ensure 20% bandwidth performance does not degrade beyond −20 dB return loss.
The conventional approach to manufacture such a spaced triple layer window is similar to that employed for pillbox windows used to vacuum seal rectangular waveguides, i.e. each ceramic disc is first brazed into a copper tube which is then lapped to the desired length. For the triple layer window, which is shown on an enlarged scale in FIG. 2, three such ceramic discs 9, 10, and 11 are brazed to respective copper tubes. The copper tubes that make up waveguide 7 are then brazed together at joints 12, 13 to form the complete assembly. Such a manufacturing approach has a significant risk of introducing tilt between the ceramics discs and consequent mode conversion when the triple layer window is mounted in multi-mode waveguide. In addition, the nature of the spaced triple layer window 8 (FIG. 1) design results in trapped volumes between the ceramic layers. If these volumes are not vented by some means, they may lead to failure of the vacuum bond during subsequent processing of the window assembly.
An alternative approach that has been employed is to sandwich accurately machined copper cylinders between the ceramic layers and place the entire assembly within an outer copper tube, such that the ceramic layers appear to the microwave signal as being set in recesses in a copper tube. Unfortunately for waveguides with a large number of possible propagating modes the differential expansion between the copper and ceramic materials require significant recess depths to be employed, which degrades the microwave performance of the window assembly such that the 20% bandwidth with a return loss of better than −20 dB cannot readily be achieved.